The FGS-04e is a low noise 3-axis fluxgate sensor. It extends the frequency range for MT measurements down to DC. The sensor core unit is a Bartington Mag-13 MCL
For lower frequencies all transfer functions are linear. This sensor is designed for crustal studies where periods of 10,000 seconds and more are needed. Recording time for those periods are three to eight weeks depending on the data quality (magnetic activity and external interferences by man made noise).
The fluxgate is measuring the total magnetic field (B). A induction coil is measuring dB/dt.
A good coincidence can be seen for periods greater than 30s. For higher frequencies (smaller periods) a fluxgate can not be used for MT.
The 4 Hz low pass filter on the LF board can be used for this type of sensor.
If the sample rate is too high, the data looks strange:
Coming to lower sampling rates, you start to see the fluxgate data:
The fluxgate has constant transfer function: divide the spectra by 10E-4. Phase shift is zero for f < 1Hz.